1. Field of the Invention
This invention relates generally to linear differential amplifier circuits and, more particularly, to the reduction of input offset voltage and noise in the linear differential amplifier circuits.
2. Discussion of the Related Art
An important consideration in the design and use of linear differential amplifiers has been the noise generated within the amplifying device itself. This noise can take the form of "popcorn" noise or can be noise caused by aging, temperature dependence, or other phenomenon resulting in the instability of the characteristics of the amplifying circuit. In order to reduce the noise and more particularly, to reduce the inaccuracies in an amplifying device resulting from drift or aging, it has been known in the past to convert a low frequency input signal of the linear amplifier to a relatively rapid periodic signal and to reconvert the periodic signal to a low frequency signal at the output terminal of the amplifying device. This type of amplifier, referred to as a chopper-stabilized amplifier, is particularly effective in reducing the long term drift features of the amplifying device. The increase in amplifier stability is the result of operating the amplifying device in a relatively linear dynamic range, the dynamic range remaining linear and stable even though the steady operating characteristics can change.
Chopper-stablized amplifiers suffer from chopper-related noise introduced by interrupting the input signal's full amplitude. Compensation for offset voltages can be accomplished by auto-zero techniques in which a feedback circuit supplies an appropriate signal to the amplifier input terminal to compensate for the output signal component resulting from the offset voltage. This technique is relatively complex and is expensive to implement.
The use of a differential amplifier is an effort to balance certain characteristics of the amplifier device. Appreciable effort in the design of operational differential amplifier has been undertaken to provide electronic compensation for component characteristics of the amplifying device.
A need has therefore been felt for an amplifier circuit that can provide normal linear amplifier operation while reducing various types of noise normally associated with the linear amplifier.